Re: [thredds] WMS and F-TDS virtual datasets [was: Re: aggregation / arithmetic between variables]

Hi Jon,

Thanks for the code link and information.  Unfortunately I'm still confused.

It seems that you want to distinguish between reading local data files and OPeNDAP data sources and distinguish between uncompressed and compress local files. Makes sense to want to know this and to code accordingly. However, Ethan said that TDS promotes the underlying FileTypeId from the data files when returning the FileTypeId for a TDS aggregation. And you suggest I do the same for my virtual data sets. It seems to me that this will give you exactly the wrong information for how you want to classify the data. An F-TDS data source is by definition an OPeNDAP data source, but the data type of the underlying data will most of the time be a local netCDF file or type netCDF.

So if I follow the suggestion and return won't you get wrong optimization by looking the FileTypeId?


On 04/29/2011 03:45 AM, Jon Blower wrote:
Hi Roland,

The fileTypeId is used by ncWMS to decide on what algorithm to use to extract 
data.  Compressed data (e.g. NetCDF4) and data read over OPeNDAP, have very 
different performance characteristics to uncompressed, local data (e.g. 
NetCDF3, HDF4).

See the code here:

So I guess the fileTypeId of your virtual dataset should match the underlying file type.  
If this isn't easy, then from the WMS point of view you can put any old string as the 
fileTypeId and the WMS will be conservative and won't assume that data-reading is 

This is an example of the adage "all abstractions are leaky"... performance 
concerns are notorious for messing up nice clean abstractions.



Message: 1
Date: Wed, 27 Apr 2011 17:09:13 -0500
From: Roland Schweitzer<Roland.Schweitzer@xxxxxxxx>
To: thredds@xxxxxxxxxxxxxxxx
Subject: Re: [thredds] WMS and F-TDS virtual datasets [was: Re:
        aggregation / arithmetic between variables]
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Jon and Ethan,

Help me understand the best way forward with these FileTypeId in the IOSP.  
Questions below...

On 04/15/2011 03:37 PM, Ethan Davis wrote:
Hi Roland,

What should these [getFileTypeId] values be, Ethan?  Is there an
official enumeration I can reference for know values for these?
I just grabbed them off the Web page:

It makes sense to me to use netCDF since it is the intent of the IOSP
to act like netCDF OPeNDAP in every case.
The ID values should uniquely identify the "file type". The web page
enumerates the values we know. We encourage everyone that implements
an IOSP to select an ID not on the list and let us know so we can
update the list.
So, I think rather than use "netCDF" you should decide on an ID unique
to your IOSP. Or, if all the datasets behind one of your virtual
datasets are always going to be the same type, you could use the type
of the backing datasets. (That is what the CDM Aggregation class does,
it uses the "file type" of the aggregations "typical dataset").
Jon,  what does the ncWMS do with the FileTypeId?   The best decision
from my point of view for what to return seems to depend on how the value is 
being used by clients.  I thought the point of the the CDM was everything looks 
like netCDF.  If folks are making optimizations based on the FileTypeId then 
for some cases like F-TDS it seems like they might miss out if the FileTypeId 
was something other than netCDF.

Let us know if you decide on a new unique ID and we'll add it to the list .

End of thredds Digest, Vol 27, Issue 34

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